The present invention relates to a process for preparing ethanebis(methylphosphinic) acid from methylphosphinic acid, and also to the use of the products prepared by this process.
Phosphinic acids and salts of these may be prepared by a variety of methods, and have been described widely within the literature.
Organic phosphinic acids, and their salts and esters are known flame retardants. For example, EP 0 699 708 A1 describes flame-retardant polyester molding compositions, these being rendered flame-retardant by adding the calcium or aluminum salts of phosphinic or diphosphinic acids. The abovementioned salts are obtained by reacting the corresponding phosphonic acids with calcium hydroxide or aluminum hydroxide.
Due to their high phosphorus content and especially their bidentate nature, the diphosphinic acids are described as highly effective reactive flame retardants for polyesters, e.g. for textile applications. This particularly applies to ethanebis(methylphosphinic) acid, specifically in the form of its glycol ester (DE 22 36 037 A1).
The preparation of ethane(methylphosphinic) acid is technically very complicated, an example of the procedure being an Arbuzov reaction of diisopropyl methylphosphonous diesters with ethylene bromide [P. Mastalerz, Rocziniki Chem 38 (1964), pp. 61-64], followed by ester cleavage. The phosphonous diester used is prepared from methylphosphonous dichloride by reaction with alcohols.
Another way of preparing ethanediphosphonic acids is proposed in DE 23 02 523 A1 by reacting alkylphosphonous esters with ethyne (acetylene) and then cleaving the diester with HCl, with formation of alkyl chlorides. Here again, the alkylphosphonous esters used are prepared from the corresponding phosphonous dihalides by hydrolysis and reaction with alcohols.
The abovementioned reactions are very difficult to carry out, since their final step is cleavage of the corresponding esters, and this gives technical difficulties.
In addition, there are byproducts formed which, like some of the abovementioned starting materials, are toxic, or ignite spontaneously, and/or are corrosive, i.e. are highly undesirable.
The object on which the invention is based is therefore to provide a process which can prepare ethanebis(methylphosphinic) acid and is particularly easy and economic to carry out and gives high yield of a single product, and in particular dispenses with the complicated cleavage of diphosphinic esters. This process should also be clearly superior to the known processes in its effect on the environment.
This object is achieved by way of a process for preparing ethanebis(methylphosphinic) acids which comprises reacting ethyne (acetylene) with methylphosphinic acid: 
Compared with the processes known hitherto, the process of the invention has considerable advantages since it produces no halogenated organic byproducts, involves no complicated cleavage of phosphinic esters, and also has a positive balance in relation to product distribution. The process is highly effective and economic to carry out.
Methylphosphinic acid is prepared in a simple and known manner by hydrolyzing methylphosphonous dichloride.
The methylphosphinic acid is preferably reacted with ethyne(acetylene) in the presence of a free-radical initiator.
The free-radical initiators used preferably comprise azo compounds.
The azo compounds are preferably cationic and/or non-cationic azo compounds.
The cationic azo compounds used preferably comprise 2,2xe2x80x2-azobis(2-amidinopropane) dihydrochloride or 2,2xe2x80x2-azobis(N,Nxe2x80x2-dimethyleneisobutyramidine) dihydrochloride.
The non-cationic azo compounds used preferably comprise azobis(isobutyronitrile), 4,4xe2x80x2-azobis(4-cyanopentanoic acid), or 2,2xe2x80x2-azobis(2-methylbutyronitrile).
The free-radical initiators used preferably comprise peroxidic inorganic and/or peroxidic organic free-radical initiators.
The peroxidic inorganic free-radical initiators used preferably comprise hydrogen peroxide, ammonium peroxodisulfate, and/or potassium peroxodisulfate.
The peroxidic organic free-radical initiators used preferably comprise dibenzoyl peroxide, di-tert-butyl peroxide, and/or peracetic acid.
A wide selection of suitable free-radical initiators can be found by way of example in Houben-Weyl, Supplementary volume 20, in the chapter xe2x80x9cPolymerisation durch radikalische Initiierungxe2x80x9d [Free-radical-initiated polymerization] on pages 15-74.
The free-radical initiators are preferably metered in continuously during the reaction.
The free-radical initiators metered in continuously during the reaction are preferably in the form of a solution in the ethyne(acetylene).
The free-radical initiators metered in continuously during the reaction are preferably in the form of a solution in the solvent used.
The reaction preferably takes place at a temperature of from 40 to 200xc2x0 C.
The reaction particularly preferably takes place at a temperature of from 70 to 130xc2x0 C.
The reaction preferably takes place in the presence of a solvent.
The reaction preferably takes place in acetic acid as solvent.
The reaction preferably takes place by introducing gaseous ethyne (acetylene) at atmospheric pressure.
The reaction preferably takes place at superatmospheric pressure.
The manner of conducting the process is preferably such that after partial conversion the precipitating ethanebis(methylphosphinic) acid is filtered off, and further acetylene is added after replacing the methylphosphinic acid consumed.
The present invention also provides a process in which pure methylphosphinic acid is reacted with ethyne (acetylene) in the presence of a cationic or non-cationic free-radical initiator or in the presence of a peroxidic free-radical initiator, to give ethanebis(methylphosphinic) acid.
The present invention also provides a process in which methylphosphinic acid is reacted with ethyne (acetylene) in the presence of a cationic or non-cationic free radical initiator or in the presence of a peroxidic free-radical initiator in acetic acid, to give ethanebis(methylphosphinic) acid, and this is continuously removed from the reaction mixture by a circulating filter system, and the methylphosphinic acid consumed is likewise continuously replaced by fresh acid.
The desired ethane bis(methylphosphinic) acid is obtained with high selectivity and high purity.
Either the methylphosphinic acid or the ethyne (acetylene) may be used in excess, since the reaction partners always react In a molar ratio of 2 to 1 (methylphosphinic acid to ethyne (acetylene)).
The invention also provides the use of the ethanebis(methylphosphinic) acid prepared by the process of the invention as starting material for preparing flame retardants for polymers.
The invention further provides the use of the ethanebis(methylphosphinic) acid prepared by the process of the invention as starting material for preparing flame retardants for thermoplastic polymers, such as polyethylene terephthalate, polybutylene terephthalate, or polyamide.
The invention also provides the use of the ethanebis(methylphosphinic) acid prepared by the process of the invention as starting material for preparing flame retardants for thermoset resins, such as unsaturated polyester resins, epoxy resins, polyurethanes, or acrylates.
Finally, the invention also provides the use of the ethanebis(methylphosphinic) acid prepared by the process of the invention as precursor for the chemical synthesis of other phosphorus-containing compounds.